Pheochromocytomas are tumors of neural crest origin that produce excessive quantities of norepinephrine and/or epinephrine. In recent studies of the usefulness of either the glucagon provocative test or the clonidine suppression test in the diagnosis of patients with pheochromocytoma, we have found the glucagon test to have an overall sensitivity of 81% and a specificity of 100%, while the clonidine suppression test has a sensitivity of 87% and a specificity of 93%. Once the diagnosis. is made, localization of the tumor is necessary. CT has a sensitivity of 96% and a diagnostic accuracy of 89%, MRI has a sensitivity of 100% and a diagnostic accuracy of 89%, while radiolabelled MIBG scanning has a sensitivity of 86% and a diagnostic accuracy of 89%. Thus, the MIBG scan is the most specific test for localizing a pheochromocytoma, and nearly as sensitive as either CT or MRI.